Andesite

For the extinct cephalopod genus, see Andesites.
Andesite
Igneous rock
Andesite pmg ss 2006.jpg
Photomicrograph of andesite in thin section (between crossed polars)
Composition

Intermediate
Major minerals: plagioclase (often andesine) and pyroxene or hornblende

Accessory minerals: magnetites, biotite, sphene, quartz
A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. Diameter of view is 8 cm.
Andesite Mount Žarnov ( Vtáčnik), Slovakia
Andesite pillar in Slovakia

Andesite ( t/ or t/ [1]) is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and granite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. [2] Alkali feldspar may be present in minor amounts. The quartz- feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.

Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. The average composition of the continental crust is andesitic. [3] Along with basalts they are a major component of the Martian crust. [4] The name andesite is derived from the Andes mountain range.

Generation of melts in island arcs

Magmatism in island arc regions (i.e., active oceanic margins) comes from the interplay of the subducting plate and the mantle wedge, the wedge-shaped region between the subducting and overriding plates.

During subduction, the subducted oceanic crust is submitted to increasing pressure and temperature, leading to metamorphism. Hydrous minerals such as amphibole, zeolites, chlorite etc. (which are present in the oceanic lithosphere) dehydrate as they change to more stable, anhydrous forms, releasing water and soluble elements into the overlying wedge of mantle. Fluxing water into the wedge lowers the solidus of the mantle material and causes partial melting. [5] Due to the lower density of the partially molten material, it rises through the wedge until it reaches the lower boundary of the overriding plate. Melts generated in the mantle wedge are of basaltic composition, but they have a distinctive enrichment of soluble elements (e.g. potassium (K), barium (Ba), and lead (Pb)) which are contributed from sediment that lies at the top of the subducting plate. Although there is evidence to suggest that the subducting oceanic crust may also melt during this process, the relative contribution of the three components (crust, sediment, and wedge) to the generated basalts is still a matter of debate. [6]

Basalt thus formed can contribute to the formation of andesite through fractional crystallization, partial melting of crust, or magma mixing, all of which are discussed next.

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